The present invention relates to the field of amplifiers in general, and more particularly, to power amplifiers in wireless user terminals.
Radiotelephones that operate in wireless communication systems, such as CDMA IS-95 or AMPS IS-19, may use a Power Amplifier (PA) to amplify signals transmitted to the communications system via a radiotelephone antenna. The radiotelephone may also receive signals via the antenna from the system which are provided to a receiver therein.
Some radiotelephones include a duplexer that can enable the radiotelephone to use the same antenna for the transmission and reception of the respective signals. For example, signals generated by a transmitter and amplified by the PA can be transmitted via the antenna through the duplexer and signals received via the antenna can be provided to the receiver through the duplexer. Unfortunately, noise in the signals generated by the PA may be coupled to the received signals in the duplexer. The received signals, including the noise generated by the PA, may then be provided to the receiver which can result in a reduction in the sensitivity of the receiver.
Some conventional radiotelephones reduce noise included in the signals provided to the receiver by using a PA having a low-noise specification for receive band frequencies. Consequently, some PAs which do not meet the requisite noise performance criteria, but have other desirable characteristics may be excluded from use. Accordingly, there is a need, for example, to further improve noise performance of radiotelephones.
It is, therefore, an object of the present invention to allow improved user terminals that operate within a wireless communications system.
It is another object of the present invention to allow improved noise performance of user terminals that operate within a wireless communications system.
These, and other objects of the present invention, may be achieved by a difference frequency impedance transformation circuit that changes an impedance presented to an input of a power amplifier at a difference frequency that is a function of a difference between receive and transmit frequency ranges. In particular, a difference frequency impedance transformation circuit can include a first circuit that conducts the first signals to an input of the power amplifier, and a second circuit that changes the impedance presented to the input of the power amplifier at the difference frequency, where the difference frequency is a function of a difference between the receive frequency range and the transmit frequency range. According to the present invention, reducing noise at out-of-band frequencies may result in reduced noise coupled to received signals in the duplexer and passed to the receiver. This may allow some power amplifier devices with lesser noise performance to be used while reducing the noise which may otherwise be coupled to the receiver. In contrast, in conventional systems the difference frequency may be mixed with the transmit frequencies to produce noise in the receive frequency range.
According to one embodiment, the difference frequency impedance transformation circuit provides an impedance to the input of the power amplifier at the difference frequency that is greater than the impedance provided to frequencies in the transmit range. In another embodiment according to the present invention, the impedance provided to the input of the power amplifier at the difference frequency is less than the impedance provided to frequencies in the transmit range.
In one embodiment according to present invention using AMPS-19, the transmit frequency range is about 824 MHz to 849 MHz, the receive frequency range is about 869 MHz to 894 MHz, and the difference frequency is about 45 MHz. In an embodiment according to the present invention using CDMA IS-95, the transmit frequency range is about 1850 MHz to 1910 MHz, the receive frequency range is about 1930 MHz to 1990 MHz, and the difference frequency is about 80 MHz.
According to a further aspect of the present invention, a power amplifier device includes a first power amplifier stage having a first input thereto and a first output and a second power amplifier stage having a second input that is electrically coupled to the first output, where the first signal is electrically coupled to the second input.
A difference frequency impedance transformation circuit changes the impedance presented to the input of the second stage at frequencies that are a function of the difference between the receive frequency range and the transmit frequency range. In particular, if noise is present at the output of the first stage, the second out-of-band circuit can change the impedance for frequencies about equal to the difference between the receive and transmit frequency ranges. Furthermore, the present invention may be practiced using a power amplifier device that includes more than two stages. For example, in a further embodiment, a third out-of-band circuit can be electrically coupled to input of a third stage in the power amplifier device.